In olefin epoxidation an olefin is reacted with oxygen to form an olefin epoxide, using a catalyst comprising a silver component, usually with one or more further elements deposited therewith on a support. The olefin oxide may be reacted with water, an alcohol, carbon dioxide or an amine to form a 1,2-diol, a 1,2-diol ether, 1,2 carbonate or an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers and alkanolamines may be produced in a multi-step process comprising olefin epoxidation and converting the formed olefin oxide with water, an alcohol or an amine.
The performance of the epoxidation process may be assessed on the basis of the selectivity, the catalyst's activity, and stability of operation. The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. Modern silver-based epoxidation catalysts are highly selective towards olefin oxide production. When using the modern catalysts in the epoxidation of ethylene the selectivity towards ethylene oxide can reach values above 85 mole-%. An example of such highly selective catalysts is a catalyst comprising silver and a rhenium promoter, for example U.S. Pat. Nos. 4,761,394 and 4,766,105.
For decades much research has been devoted to improving the activity, the selectivity, and the lifetime of the catalysts, and to find process conditions which enable full exploitation of the catalyst performance. A reaction modifier, for example an organic halide, may be added to the feed in an epoxidation process for increasing the selectivity of a highly selective catalyst (see for example EP-A-352850, U.S. Pat. Nos. 4,761,394 and 4,766,105, which are herein incorporated by reference). The reaction modifier suppresses the undesirable oxidation of olefin or olefin oxide to carbon dioxide and water, relative to the desired formation of olefin oxide. EP-A-352850 teaches that there is an optimum in the selectivity as a function of the quantity of organic halide in the feed, at a constant oxygen conversion level and given set of reaction conditions.
Many process improvements are known that can improve selectivity. For example, it is well known that low CO2 levels are useful in improving the selectivity of high selectivity catalysts. See, e.g., U.S. Pat. Nos. 7,235,677; 7,193,094; US Pub. Pat. App. 2007/0129557; WO 2004/078736; WO 2004/078737; and EP 2,155,708. These patents also disclose that water concentration in the reactor feed should be maintained at a level of at most 0.35 mole percent, preferably less than 0.2 mole percent. Other patents disclose control of the chloride moderator to maintain good activity. See, e.g., U.S. Pat. No. 7,657,331; EP 1,458,698; and U.S. Pub. Pat. App. 2009/0069583. Still further, there are many other patents dealing with EO process operation and means to improve the performance of the catalyst in the process. See, e.g., U.S. Pat. Nos. 7,485,597, 7,102,022, 6,717,001, 7,348,444, and U.S. Pub. Pat. App. 2009/0234144.
All catalysts must first be started up in a manner to establish a good selectivity operation. U.S. Pat. No. 7,102,022 relates to the start-up of an epoxidation process wherein a highly selective catalyst is employed. In this patent there is disclosed an improved start-up procedure wherein the highly selective catalyst is subjected to a heat treatment wherein the catalyst is contacted with a feed comprising oxygen at a temperature above the normal operating temperature of the highly selective catalyst (i.e., above 260° C.). U.S. Pub. Pat. App. 2004/0049061 relates to a method of improving the selectivity of a highly selective catalyst having a low silver density. In this document, there is disclosed a method wherein the highly selective catalyst is subjected to a heat treatment which comprises contacting the catalyst with a feed comprising oxygen at a temperature above the normal operating temperature of the highly selective catalyst (i.e., above 250° C.). U.S. Pat. No. 4,874,879 relates to the start-up of an epoxidation process employing a highly selective catalyst wherein the highly selective catalyst is first contacted with a feed containing an organic chloride moderator and ethylene, and optionally a ballast gas, at a temperature below the normal operating temperature of the catalyst. EP-B1-1532125 relates to an improved start-up procedure wherein the highly selective catalyst is first subjected to a pre-soak phase in the presence of a feed containing an organic halide and is then subjected to a stripping phase in the presence of a feed which is free of the organic halide or may comprise the organic halide in a low quantity. The stripping phase is taught to continue for a period of more than 16 hours up to 200 hours. U.S. Pat. App. No. 2009/0281339 relates to the start-up where the organic chloride in the feed is adjusted to a value sufficient to produce EO at a substantially optimum selectivity.
At the end of the start-up period, the chloride level is typically adjusted to find the chloride level which gives the maximum selectivity at the desired EO production rate. The plant then sets the chloride level equal to this so-called “chloride optimum” and begins normal operation of the catalyst, which continues until it is discharged from the reactor. During normal operation of the catalyst, several routine things may happen:                The catalyst will deactivate. In order to maintain a constant production rate, the reaction temperature will be increased as the catalyst deactivates.        The production rate may change, due to feedstock availability, production demands, or economics. To increase the production rate, the reaction temperature will be increased; to decrease the production rate, the reaction temperature will be decreased.        The feed composition may change. Generally, CO2 levels will increase over the life of the catalyst as selectivity drops. Also, ethylene and oxygen levels may be changed due to feedstock issues or to lower temperature near end-of-cycle.        Feed impurities (such as ethane or propane) may fluctuate.        There may be an upset in operation due to such events as equipment failure or unplanned operation changes or deviations from normal operation.        
It is well-known (see, e.g., U.S. Pat. No. 7,193,094 and EP 1,458,698) that changes in reaction temperature or hydrocarbon concentration will change the chloride optimum. For example, as the reaction temperature increases or as hydrocarbon levels increase, the chloride level will also need to be increased in order to maintain operation at the maximum selectivity. During routine plant operation, the chloride level is adjusted in one of two methods:                1. The plant utilizes some proprietary mathematical formula which relates chloride level to temperature, composition, etc. This formula is computed periodically and if the chloride level is found to be significantly different than the optimal level (as determined by the formula), then the chloride level is adjusted so that it equals the optimal level.        2. More frequently, the plant routinely checks whether the chloride level is still optimized. This may happen at some fixed frequency or following certain changes in operating conditions, as determined by the plant. Typically, the chloride level is increased or decreased slightly and the plant observes whether the selectivity changed. If it did not change, then they were probably operating at the selectivity maximum, so the chloride level is reset to its original value. If the selectivity did change, then the chloride level is changed in small steps until a selectivity maximum is found, and then the plant continues operation at this new chloride optimum.        
Notwithstanding the improvements already achieved, there is a desire to further improve the performance of the silver-containing catalysts in the production of an olefin oxide, a 1,2-diol, a 1,2-diol ether, 1,2-carbonate or an alkanolamine.